It was almost a footnote. AirPlay, the audio streaming protocol once known as AirTunes, got just one minute of keynote time last week. But it might end up as the backbone of Apple's assault on the living room.

What Does AirPlay Do?

At its most simple, AirPlay streams music from one Apple device to another. Extending the potential of its predecessor AirTunes, AirPlay will also stream video, photos, and metadata.

In short, connect your iPhone to your Apple TV and you can watch movies from your iPhone on your television. Connect your iPad to your stereo and play Pandora on your nice speakers. Load photos from your iPod Touch to an AirPlay-compatible digital photo frame.

While the types of media you can shoot around will definitely be constricted by Apple's closed ecosystem, the number of devices that will be able to speak AirPlay's language are more or less limitless—provided approved manufacturers pay a license fee to Apple.

How Does AirPlay Work?

There's a long history of Apple music sharing, starting with the device discovery service we now known as Bonjour almost ten years ago. Bonjour lets compatible devices on the same network say hello. "I'm here," says each computer,"And here's what I can share with you."

Bonjour is the reason you can see all those other laptops when you're sharing a coffee shop's Wi-Fi network or other students using a dorm's Ethernet network. In general, Mac OS X users use Bonjour to share files or iTunes libraries—or more accurately, they use Bonjour to let other computers know what protocols to use to share things, such as the Digital Audio Access Protocol used by iTunes to share its music library with computers and audio streamers.

AirTunes sends an encrypted stream of packets that sync up every couple of seconds, as well as some basic controls—play, pause, skip a track, the like. Although there aren't actually any AirPlay devices out yet, it's pretty reasonable to presume that they use the same (or relatively) protocol. Except now AirPlay can also send video, photos, and related metadata like titles, artist info, and playcounts between any AirPlay-compatible device.

Apple has partnered with a company called BridgeCo to provide the hardware that other electronics companies can use to integrate with AirPlay. Previously, the only real AirTunes hardware was Apple's own AirPort Express Wi-Fi base station (and other Macs). One nice perk with partnering with BridgeCo: Their streaming system—basically some chips and a software platform wedded to Wi-Fi—also supports other standards like UPnP

When Steve Jobs demoed AirPlay at the tail end of the new Apple TV presentation, he moved a movie, Pixar's Up, from an iPad's screen to a television with an attached Apple TV. The iPad itself displayed a television icon in the Video app instead of the video, while onscreen controls still functioned.

While we can't know exactly how AirPlay video will work until we actually have devices in hand that support it—and the iOS 4.2 update that's coming in November—it appears that it's actually streaming the content from the iPad to the AppleTV, not passing on a token to the Apple TV, authorizing it stream directly from Apple's servers. Perhaps it does do that—or will in a future update—but the upshot is the same for now: You can "beam" content from any support iOS device to any AirPlay device, but you won't be able to use your iOS device for other media playback. (Hopefully other functions like web browsing and apps will work while the content streams in the background.)

Why AirPlay Isn't As Closed As It May Seem

Andrew Orlowski is worried that AirPlay is just another Apple attempt to sell a closed version of something that was previously open. That's Apple's iOS (and before that, iPod) business model.

But the difference this time is BridgeCo, the company with which Apple developed AirPlay. They've been around for years developing a streaming media solution that's already being used by hardware manufacturers.

BridgeCo CEO Gene Sheridan told me this morning that the company really only makes one product, the "Jukeblox Platform." JukeBlox isn't the sort of thing you're going to see on a product bullet list or in a sticker on the side of a box at Best Buy. What you will see on those bullet lists are the things that JukeBlox enables: media streaming capabilities in a variety of formats, including UPnP, DNLA, and Windows 7 streaming.

That means that all AirPlay-enabled hardware also supports the industry's previously most popular streaming formats. So if you're the sort of person who wouldn't use an Apple product if your life depended on it, it's possible that more AirPlay products will mean more options for your preferred media streaming platform.

Even better, at least one manufacturer has already announced a firmware update that will add AirPlay support to a product already on the shelves. It's hard to know if a company is selling a product already using BridgeCo's hardware (and thus able to be upgraded by software to AirPlay), but hopefully those that are will release new firmware instead of leaving older customer behind.

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Tantalizingly, this also means that in the current implementation of JukeBlox, software updates are relatively trivial. That means that it may be possible for Apple to extend the capabilities of AirPlay—screen sharing, for instance—without fracturing the support among older devices.

The wild card, as usual, is Apple's willingness to play nice with non-Apple standards. Since Apple has to approve the licensing for every AirPlay certified device—just like the lucrative "Made for iPod" scheme of years past—they could, in theory, require manufacturers to disable any streaming protocol except AirPlay.

I think Apple might refrain for two reasons: Third-party manufacturers still have to sell their hardware—the more capability a device has, the better—so I expect there may be some push back if Apple becomes too covetous; Device-to-device streaming isn't exactly an everyday occurrence for most consumers. Apple, with millions of iOS devices already in hands of customers, might not need to disrupt other streaming standards if AirPlay becomes the dominant format quickly. History implies a better than average chance.

Still, it will be hard to fight Apple if they turn the screws. As BridgeCo's Gene Sheridan said to me, "Just to participate in the Apple ecosystem is a big opportunity by itself." There are companies out there making millions just by selling Apple accessories. Being first in the AirPlay market—docks for content—might be worth billions.

What a Future With AirPlay Looks Like

Did you know iTunes 10 lets you plug any iPod or iPhone into anyone's computer and play music right off the device? (It used to try to sync with iTunes, causing much consternation.)

That's no accident. Taken along with the parts of AirPlay already shown, it's becoming clear that Apple is becoming less concerned with treating the media on its devices as something they must carefully guard. Granted, they want you to buy or rent that media from them. (That's one of my biggest questions about AirPlay: Will I be able to push non-DRM content around without issue? What about video from players other than Apple's? What about content from Macs?)

But it looks like Apple is setting up the possibility to share the content on your iOS device with just a couple of clicks, not just at home, but anywhere.

Want to show your friends your vacation photos? As long as you're on their Wi-Fi network, you should be able to throw them up on their HDTV via Apple TV. (Or directly to the TV, once someone makes an AirPlay-licensed HDTV.)

Want to watch an episode of Mad Men you just rented again at your girlfriend's house? Hit play on your iPhone and send it the digital photo frame that sits on her desk while you play Angry Birds.

Or maybe it's a party, the host's music sucks, and you want to play your own mix on their speakers. Should be easy.

Of course, Apple could—and should—put a pairing passcode on AirPlay devices. But they should also allow an "open" mode for AirPlay devices akin to an open Wi-Fi network. If someone is already on your Wi-Fi network, it's fair to presume you can hunt them down and sock them in the jaw if they keep playing awful music on the speakers in your bathroom—once you get off the can.

Apple TV Is A Hobby, But AirPlay Isn't

More than anything, it's clear that Apple TV is only a hobby because that's all Apple needs it to be to win in the living room. Sure, it has a simple interface with a simple remote; no need to make it completely useless without an iOS device—just better when you do. But in the end it doesn't need to be anything than just another AirPlay-compatible device in a world where the rest of the world's electronics speak Apple.